New instrument for quantitative measurements of passive duction forces and its clinical implications

Purpose: Evaluating the passive duction force of the extraocular muscles is important for the diagnosis of and surgical planning for strabismus. This is especially relevant in patients with an observable limitation of duction movement. The purpose of this study was to validate passive duction forces in healthy subjects using a novel instrument. Methods: An instrument for making continuous quantitative measurements of passive duction forces was designed. Tension was measured as the eyeball was rotated horizontally or vertically from the resting position under general anesthesia 10 mm (50°) away from the direction of force to be tested (opposite side). Results: Seventy eyes of 35 subjects were enrolled in this study (age range of 4–80 years and mean age of 36.3 years). The passive duction force was measured at 49.0 ± 15.3 g (mean ± standard deviation) for medial rotation, 44.8 ± 13.2 g for lateral rotation, 50.5 ± 14.8 g for superior rotation, and 53.5 ± 13.8 g for inferior rotation. The passive duction forces were similar for all gaze positions, but it was larger for inferior rotation than for lateral rotation (P = 0.009). The passive duction force was significantly larger for vertical rotation (51.9 ± 14.4 g) than for horizontal rotation (46.9 ± 14.4 g) (P = 0.006). The passive duction force did not differ significantly with sex (P = 0.355), side (P = 0.087), or age (P = 0.872). Conclusions: These measurements of passive duction forces in a healthy population provide valuable information for diagnosing specific strabismic problems and could be useful for increasing the precision of strabismus surgery. [Figure not available: see fulltext.]